This is exactly what CSS was designed for: allowing you to define your personal style preferences in your browser, applying them across all websites. The term ‘cascading’ reflects this purpose.
Unfortunately, the web today has strayed far from its original vision. Yet, we continue to rely on the foundational technologies that were created for that very vision.
IMO browsers are broadly dropping the ball and failing to be "the user's agent." Instead they are the agents of web developers, giving them the powers that users should have.
If browsers catered to their user's desires more than they cater to developers, the web wouldn't be so shitty.
This is going to be an unpopular opinion, but I think the beginning of the end was the invention of JavaScript. Pulling down an unknown chunk of code from the internet and running it is malware. Even if browsers successfully sandbox the JS (a promise which they've failed to keep numerous times) it can do all sorts of stuff that doesn't serve me, like mine crypto (theft of resources) or display ads (adware).
The primary benefit of web applications is they don't lose your data. Not a single web application UI that exists provides as good a user experience as the native desktop applications that came before. A web where browsers provided their own UIs for various document types, and those document types could not modify their UIs in any way, period, would be a better web. You serve up the document, I get to control how it looks and behaves.
Unfortunately, the web today has strayed far from its original vision. Yet, we continue to rely on the foundational technologies that were created for that very vision.